Field of the Invention
The invention lies in the field of electronic signal processing. More specifically, the invention relates to a method for estimating a direction of incoming wave elements of a received signal, for example in base stations for mobile radio networks or in applications for radar or sonar systems and for seismic measurement systems.
One or more subscriber signals can be caused by transmitting from one or more communication subscribers to a common receiving station, by transmitting from one or more transmitters with superimposed measurement signals or by reflections of a measurement signal on obstructions or geological layers.
Estimation methods for determining the directions of arrival of various signals are described by Roy and Kailath, "ESPRIT--Estimation of Signal Parameters via Rotational Invariance Techniques," IEEE Trans. Acoustics, Speech, and Signal Processing, Vol. ASSP-37, pp.984-95, July 1989. For example, a direction estimation method which is known from German patent application DE 195 11 752 and used as the UNITARY-ESPRIT method leads to the direction of wave elements being determined directly from the received signals.
Direction estimation is very important, by way of example, in the context of mobile radio application and the novel method of direction estimation will be discussed in the following in the context of mobile radio.
With mobile radio or methods similar to mobile radio, a new field of application has been opened up for direction estimation. When signals propagate in a propagation medium, they are subject to interference caused by noise. Owing to diffraction and reflections, signal components pass through different propagation paths and are superimposed at the receiver. This leads to superposition cancellation effects. Furthermore, if there is more than one signal source, this leads to the signals being superimposed. Frequency-division multiplex (FDMA), time-division multiplex (TDMA) or a method which is known as code division multiplex (CDMA) are used to distinguish between the signal sources, and thus to evaluate the signals.
If, for example, a CDMA (code division multiple access) method is used for subscriber separation, a plurality of subscriber signals can be transmitted in one frequency channel at the same time. The individual signals are separated in the receiver.
Mathematical descriptions, the method of operation, and the structure of CDMA (Code Division Multiple Access) radio transmission systems are described by Jung and Blanz in "Joint Detection With Coherent Receiver Antenna Diversity in CDMA Mobile Radio Systems," IEEE Transactions on Vehicular Technology, Volume VT-44, 1995, pages 76-88. When such systems are used for mobile communication, there is a radio interface between fixed-position base stations and moving mobile stations. The transmission path from a base station to a mobile station is called the downlink path, and the transmission path from a mobile station to a base station is called the uplink path.
The Jung and Blanz article furthermore shows that the transmission quality in such radio transmission systems can be improved since it is possible to use an arrangement of a plurality of receiving sensors instead of a single receiving sensor. In accordance with the terminology used in the above-mentioned document, K denotes the number of subscriber signals which are transmitted from a base station at the same time in the same frequency channel, for example of supplied mobile stations. Ka denotes the number of receiving sensors which are assigned to a receiving device, for example the base station. In one such scenario, there are, therefore, K--Ka radio channels in the uplink path between the K mobile stations and the Ka receiving sensors in the base station. Each of the radio channels is characterized by a discrete-time baseband equivalent of its channel pulse response g.sup.(k) (ka) where k=1 . . . K, ka=1 . . . Ka. These channel impulse responses g.sup.(k) (ka) are used for channel modeling in data detection. That method does not take into consideration any statements relating to the directions in which wave elements arrive.
International PCT publication WO 95/09490 describes a mobile radio system with spatial subscriber separation, in which two different channel classes with a different capacity are used. After determining the position of the mobile station, narrow directional polar diagrams are used for transmission.
European Patent Application EP 0 701 334 A2 discloses a method for determining impulse responses of a radio channel in a cellular radiotelephone system.